1. Technical Field
The present invention relates to the field of on-load tap-changer technologies, and particularly to a change-over selector with a vacuum arc extinguishing circuit.
2. Related Art
For an on-load tap-changer, in the case that a transformer is on load, that is, is uninterrupted, a turn ratio of a primary winding to a secondary winding of the transformer is changed by changing winding taps connected to the transformer, so as to achieve the purpose of changing an output voltage of the transformer.
The existing combinational on-load tap-changer is divided into two parts, that is, a change-over switch and a selector. For some transformers, a voltage regulating range is required to be large, and the number of stages is large. A change-over selector can be added in the selector part. A polarity of a voltage regulating winding of the transformer is changed over or connection of a coarse or thin regulation winding is changed, so as to achieve the purpose of expanding the voltage regulating range of the transformer without increasing the taps of the voltage regulating winding of the transformer.
The change-over selector changes over the polarity of the voltage regulating winding of the transformer or changes the connection of the coarse or thin regulation winding, that is, a coarse or thin regulation change-over selector and a polarity change-over selector. The operation can only be performed at a middle position of the whole voltage regulating range, that is, moving contacts of the coarse or thin regulation change-over selector and the polarity change-over selector perform coarse or thin regulation and polarity change-over regulation at a K position.
The operation of the polarity change-over selector shown in FIG. 1A, FIG. 1B, and FIG. 1C is performed on a tapped “middle position”, that is, the tap selector is located in a K position. When the polarity is changed over, at the moment that the moving contact leaves “+” or “−”, the voltage regulating winding of the transformer is separated from the primary winding, and the voltage regulating winding is in an electrical “floating” state.
The operation of the coarse regulation change-over selector shown in FIG. 2A, FIG. 2B, and FIG. 2C is performed on a tapped “middle position”, that is, the tap selector is located in a “K” position. During change-over, at the moment that the moving contact leaves “+” or “−”, the voltage regulating winding of the transformer is separated from the primary winding, and the voltage regulating winding is in an electrical “floating” state.
Since a voltage exists on an winding adjacent to the voltage regulating winding and coupling capacitance exists between the voltage regulating winding and the adjacent winding and between the voltage regulating winding and an adjacent grounding body, at the moment of separation, a new potential different form a potential of the voltage regulating winding before the actions of the coarse regulation change-over selector and the polarity change-over selector occurs on the voltage regulating winding, and the moving contacts of the coarse regulation change-over selector and the polarity change-over selector are still on the original potential. The potential difference between two potentials is called a displacement voltage (or called an offset voltage). The voltage acts on a clearance between the moving contact of the coarse regulation change-over selector or the polarity change-over selector and a “+” or “−” static contact at the same time, so the voltage is called a recovery voltage. When a value of the recovery voltage exceeds the endurance of the change-over selector, continuous discharging may occur on the clearance of the change-over selector.
The intensity of the capacitance current and recovery voltage depends on a system voltage, a coil arrangement manner, and capacitance and a ratio between coils. For a given coil arrangement manner and coil capacitance, the capacitance current cannot be reduced.
In order to limit the recovery voltage, the prior art generally adopts the following solutions:
1. A fixed potential resistor RP is mounted between a middle portion of a voltage regulating coil and a change-over switch leading-out terminal (referring to FIG. 3). In this method, the structure is simple, but the potential resistor RP is conductive for a long term, which wastes power, plays a role of increasing the temperature of the transformer oil, and is not beneficial for running of the transformer.
2. Referring to FIG. 4A, FIG. 4B and FIG. 4C, a potential resistor R is concatenated to a potential switch M linked to a moving contact. The potential switch M is connected to the potential resistor R only during the change-over operation. The purpose of using the potential switch M is to avoid many potential resistors, thereby simplifying the mounting of the potential resistor and reducing the reactive compensation. In this method, the structure of the change-over selector is complex.
3. Change-over selection without interruption is implemented by using a dual polarity change-over principle as shown in FIG. 5A, FIG. 5B, and FIG. 5C. The basic principle is as follows. In the change-over selector, a pair of static contacts 1, 1a and a pair of moving contacts 2, 2a are used to implement change-over selection. In the change-over process, the moving contact 2 is gradually switched from contact with the static contact 1 to contact with the static contact 1a, and the moving contact 2a is gradually switched from contact with the static contact 1a to contact with the static contact 1, that is, in the change-over process of the moving contacts 2, 2a, the electrical “floating” does not occur. Therefore, the recovery voltage problem does not exist, that is to say, spark discharge does not occur. However, in the dual polarity change-over selector designed by using the principle, a drive mechanism needs to be additionally configured, synchronization with the on-load tap-changer needs to be taken into consideration, and the mechanical structure is complex; moreover, the contact group of the no-load tap-changer is arranged in a panel, a mounting position needs to be left on the transformer, and a control system and an operating system need to be provided, thereby increasing the cost of the device, and increasing the volume and the oil consumption of the transformer.